1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus for performing image processing based on image data or drawing commands input from an image processing terminal.
2. Description of the Related Art
Recently, output devices such as color printers with high accuracy have become common and in the field of DTP that requires printing output with great accuracy, such output devices are used to, for example, to make proofs of page layouts and the like prepared through creation, editing and processing on image processing terminals such as personal computers, work stations and so on.
In such color printers, screen processing is performed for tone reproduction. By the screen processing, multi-valued data is converted into binary data, thereby obtaining printing output with gradation levels.
In screen processing, screen frequency is made to become relatively low (the number of screen lines is decreased) when an image is to be reproduced with smooth gradation by increasing the number of gradation levels. Screen frequency is made to become relatively high (the number of screen lines is increased) when thin-line reproducibility of intermediate gradation-levels is to be improved.
Also, in normal color printing, although process colors of C (cyan), M (magenta), Y (yellow) and K (black) are used, ink of a color (spot color) other than the process colors may be used. Further, a document including an image (object) using the process colors and an object using the spot color may be made.
When such a document is proofed by using a printer (color comprehensive layout output), a method is possible in which a spot color plate is generated in addition to color plates for each of the process colors, and therefore, in the case of a document using spot color, proper color comprehensive output can be easily obtained.
Generally, the number of screen lines is switched for each document. Recently, however, processing has been performed in which, on a single page layout, for objects such as characters the thin-line reproducibility is improved by increasing the number of screen lines (raising screen frequency), and for objects such as photos smooth gradation is maintained by decreasing the number of screen lines (lowering screen frequency).
In addition, to improve color reproducibility of each object in a printout, there is a proposal that color is changed for each object, such as characters and pictures, and the color processing most suitable to each object is performed to print out the result.
In a color printer, in order to make print output close to that displayed on a display, color correction processing is generally carried out. In this color correction, the color value of each pixel is adjusted based on adjustment amounts specific to each printer.
The images printed out on color printers include photographic image data taken with digital cameras, and photograph image data read in from negatives, such as by film scans.
In digital cameras and the film scans if analog signals corresponding to the photographic images are formed into image data by digital processing, and then image data like this is simply displayed on a display as it is, then the quality of the image can be inappropriate and an oddness in the colors and the like can develop.
Because of this, there are proposals to correct imbalances and standardize abnormal distributions (removing bias) to provide image data of preferable image quality. Here, for example, photographic image data that has been obtained by digital processing would be analyzed for characteristic values of image brightness and contrast, color balance and the like. When there is bias in the characteristic values when the digital data is obtained (e.g. at the time of photographing) and it is determined that these are adverse then these would be subjected to correction processing by using a prescribed LUT.
However, because in a color printer color correction is made for each pixel, correction based on the characteristic values of photographic images as a whole becomes difficult.
Also, in applications for making page layouts, when photographic images and the like are subject to PDL conversion then image data can be broken up into arbitrarily sized portions. Because of this, at the time of printing out, if adjustments to the brightness, contrast, color balance and the like are made based on analysis of characteristic values for each broken up portion of image data, then rather than improving the quality of the finished photographic image output it can actually make it worse.
Because of this, in order to avoid such a problem of mismatches in the finished photographic images printed out, there is a proposal to carry out the correction processing for one page of data all at a single time.
However, in expanding the drawing command, if a flag is set so as to carry out color processing that varies by pixel, to do this it is essential to change critical parts of an image processing apparatus including a drawing command expanding part, a part writing to the image memory and the like. For this reason, it means there is a need to change critical parts and make extensive changes to a normal image processing apparatus, thereby leading to substantial development costs, and so such an image processing apparatus cannot be easily obtained.
Also, when there are several photographic images, not all of the characteristic values of the photographic images will be the same, and because of this, deteriorating image quality can occur, and even the deterioration of the quality of all of the images on a given page can occur.